In recent years, plastics have been used increasingly in place of glass as a cover or surface of choice for many applications such as watch faces, windows and table surfaces. The advantage of using plastic is that it is more lightweight, less brittle, stronger, and far less likely to break than glass. However, one of the drawbacks of using plastic is that its surface is not nearly as hard as glass. Consequently, it is much more prone to being scratched by hard objects or debris, such as metal, glass, sand, grit, or any other abrasive material. In addition, plastics are more prone to hazing or surface yellowing due to weathering, which is often caused by oxidation or the influence of ultraviolet light from the sun. Such discoloration also can occur through ordinary cleaning of plastic surfaces. Not surprisingly, while the surface of glass might remain clear for many years, plastic often becomes clouded, yellowed or otherwise opaque through oxidation and by other corrosive forces found in nature.
Plastic is the material of choice for making motorcycle windshields and airplane or jet windows, all of which must withstand high wind forces and pressure differentials. However, due to such high speed winds, damage to the windshields and windows is very likely.
Within the field of aviation and more particularly, in the area of airplane windows, having clear plastic windows is desirable. Commonly the windows are often scratched, yellowed, or otherwise partially opaque. Besides being merely bothersome to the passenger, such damage can also severely impair the ability of the pilot to see the landscape outside the plane, creating a danger to all aboard. However, the discomfort and negative feeling such window damage can cause to passengers should not be overlooked.
Four airline companies, American Airlines, Delta Airlines, United Airlines and Sky West Airlines, conducted a customer survey and found that the customers' single greatest complaint concerned damage to the planes' windows, and more specifically, that the windows were so scratched that the customers had difficulty seeing through them much of the time. The survey showed that some of the customers surveyed drew a psychological correlation between the poor condition of the windows and the level of maintenance of the rest of the plane. Another specific and related complaint involved customers who wished to take photographs of the scenery below through the windows, but who were unable to do so due to the poor condition of the windows. Although the scratched windows pose no real safety risk to the airline passengers, customer satisfaction and enjoyment is important to any business venture including that of common carriers like airlines.
In addition to commercial aircraft (of which there are some 13,000 in the United States alone) there are approximately 250,000 private planes in the United States and approximately 15,000 helicopters. Although the price of replacing the windows of these aircraft may vary according to the size, shape and materials in the window, it costs approximately $75,000 to completely replace the windows of a typical Lear 25-D jet.
In the military arena, where clarity of vision is a priority, there are tens of thousands of aircraft. In the “Gulf War” alone there were 1,700 U.S. military planes in action, many of which experienced window damage as a result of the high speed, low level flying in the sandy conditions of the Arabian deserts, or when simply sitting on the runway and being exposed to blowing sand. The cost to replace the canopies of various military fighter planes and bombers can be between about $20,000 to about $80,000 per aircraft.
Currently, technology exists to remedy the problem of scratching, hazing, and oxidation of plastic surfaces, particularly aircraft windows, but it is prohibitively expensive, requiring large amounts of both capital and labor. The current best method of scratch removal involves the use of a robotic polishing machine, a machine which typically ranges in cost from $500,000 to $1,500,000. Another drawback, in addition to the cost of initial investment, is that this machine can only polish the windows after they have been removed from the plane, which is a very time consuming and labor intensive process. It also usually requires the removal of several thousandths of an inch of the window surface to adequately remove the deeper scratches. This removal of material presents a problem under safety regulations which establish minimum thicknesses. Often, much of the plane's interior must be disassembled. Yet another factor is that the machine can only polish four windows per hour, and it requires two employees to operate. In addition to the high costs of both machine and labor, there is also the cost of the chemicals.
Another cost which must be considered is the “down time” required to remove the windows, polish them, and then reassemble them into the aircraft. Most commercial aircraft have many windows and it is not uncommon to take from between six and eight days to remove and polish every window of a plane. Consequently most airlines have chosen to forego this process as evidenced by the generally poor condition of commercial airline windows. In addition to the expensive robotic polishing machine method set forth above, another sanding system bearing the tradename Micro-Mesh has been used to remove the scratches of plastics. The MicroMesh sanding system (a product of Micro-Surface Finishing Products, Inc. of Wilton, Iowa) uses a series of rubber-backed sanding cloths with differing sizes of grit. The first step uses a sanding cloth having 2,400 gauge grit. In subsequent steps, sanding clothes of finer grades up to 12,000 gauge grit are used, In addition to using the rubber sanding block, a small amount of antistatic cream is applied after polishing is complete.
The drawbacks of using the Micro-Mesh system, as well as the robotic system as presently used, is that a large amount of plastic must be sanded away in order to remove even the smallest of scratches. In addition, a sizable area of plastic must be removed in order to avoid causing optical distortion of the plastic in the surrounding area where the scratch was removed. Another problem of the Micro-Mesh sanding system is the large amount of time it takes to perform each of the series of sandings for each type of pit.
What these two systems (robotic polishing and Micro-Mesh sanding) have in common is that in order to remove a scratch from the surface of the plastic, it is necessary to remove enough of the plastic surface to at least equal the depth of the scratch, and in most cases even more. To avoid optical distortion, the individual performing the scratch removing operation must possess a high level of skill and patience, which requires a high amount of training. Moreover, repeated scratch removals using these systems will greatly reduce the thickness of the plastic and destroy its desired protective properties. Reduced thickness can cause a window to fail airworthiness standards.
The U.S. Air Force has reported that because many windshields are manufactured to meet strict optics requirements, any removal of plastic or acrylic material during scratch removal can alter their optical qualities. In such cases, present scratch removal systems are essentially useless because in curing one problem (scratches) they create another (optical distortion).
In addition to airplane windows, there are hundreds of other applications which require clear, highly visual plastics, but good visibility is often hindered because of scratches, oxidation and other opaqueness due to environmental influences. For example, schools around the country are switching from glass windows to windows made of Lexan, a product of General Electric Corp., in order to make them vandal-proof. Transit districts and schools around the country are switching from glass to plastic windows in their buses for the same reasons. In addition, new federal mandates now require trains to change all of their windows to plastic in order to make them safer. Motorcycle windscreens are typically made of plexiglass, which is not glass, but a type of polymeric plastic. Skiing enthusiasts are often disappointed when their gondola or tram is badly hazed or scratched, preventing the panoramic view they come to expect. Similarly, golfer's like a clear view from their golf carts.
Other common uses for plastic materials are watch crystals, gauge faces, eyeglasses, goggles, and laser discs, such as audio compact discs, picture discs, video laser discs and computer CD-ROM and -RAM disks No practical methods are available for removing scratches from these surfaces at reasonable cost and in a timely manner.
What is needed, then, is a composition that may be used to remove defects in a plastic surface in a timely manner without removing such a significant amount of surface material such that optical distortion occurs.